Illegal immigrants are being made to work like "slaves" inside indoor cannabis farms in Scotland, which are being raided at a rate of more than one every week since the first such farms began production around a year ago.

Illegal immigrants are being made to work like "slaves" inside indoor cannabis farms in Scotland, which are being raided at a rate of more than one every week since the first such farms began production around a year ago.

In only 12 months, police have closed 66 of the industrial-scale drugs centres, which are often run by organised crime, according to Graeme Pearson, head of the Scottish Crime and Drugs Enforcement Agency (SCDEA).

The cannabis plants are being grown in homes or business properties using equipment such as lamps and fans, with each farm housing up to 1000 plants, capable of a £100,000 harvest. Recent estimates suggest most cannabis smoked in Britain is now home-grown, compared with 11% a decade ago, partly because the home-grown product is much stronger than smuggled cannabis.

Mr Pearson told Radio Scotland's Sunday Live programme yesterday: "What we have got is these types of developments within houses, factories, garages and so forth.

"In a very high number of them we have got Chinese or Vietnamese people who are locked in the premises, kept sleeping on the floor among mattresses with no visible means of support. They are virtually slaves."

The deputy chief constable said the indoor cultivation is a trend that has been seen in other countries, including Australia and Canada. He said: "Now we have it in the UK, where organised crime gangs are focusing on the industrial production of cannabis in order they can up the potency of the product."

Today, the SNP administration at Holyrood is to publish information on drug treatment and rehabilitation, showing there are 10% more people on methadone as a substitute for heroin than previously. The figures are expected to show 21,000 people being prescribed the opiate, around one-third of them living with children.

The use of the heroin substitute has become controversial, particularly since the death in 2005 of Derek Doran, a two-year-old in East Lothian who drank his mother's methadone.

Fergus Ewing, the minister responsible for community safety, wrote in the Sunday Herald yesterday: "It serves no purpose to be simplistically anti-methadone. Methadone can stabilise lives, and it has a positive impact on the lives of many people. However, methadone must also come with genuine rehabilitation."